Means and method for depositing recrystallized ferroelectric material

ABSTRACT

RECRYSTALLIZED FERROELECTRICAL MATERIAL IS DEPOSITED BY SPUTTERING ON A HEATED SUBSTRATE BY APPLYING RADIO-FREQUENCY VOLTAGE TO A TARGET HOLDER SUPPORTING A TARGET OF FERROELCTRIC MATERIAL HELD IN PROXIMITY TO THE SUBSTRATE WITHIN A CHAMBER WHICH IS EVACULATED EXCEPT FOR THE PRESENCE OF A SMALL AMOUNT OF INERT GAS, AND OPTIONALLY WITH A SMALL AMOUNT OF OXYGEN. THE RADIO-FREQUENCY VOLTAGE CAUSES IONS OF THE GAS TO STRIKE THE TARGET MATERIAL WITH SUCH FORCE AS TO BREAK OFF MOLECULES OR ATOMS WHICH BOND THEMSELVES TO THE SUBSTRATE. THE TARGET HOLDER IS PROVIDES WITH FLUID COOLANT CHANNELS WHICH PROTECT THE TARGET FROM DETERIORATION.

A. A. SNAPER MEANS AND METHOD FOR DEPOSITING RECRYSTALLIZEDFERROELEGTRIC MATERIAL 2 Sheets-Sheet 1 Filed Sept. 15, 1972WIIIIIIIIII,

'4 Hal f A. A. SNAPER Sept. 24, 1914 MEANS AND METHOD FOR DEPOSITINGRECRYSTALLIZED FERROELECTRIC MATERIAL 2 Sheets-Sheet 2 Filed Sept. 15,1972 United States Patent 3,838,031 MEANS AND METHOD FOR DEPOSITINGRECRYS- TALLIZED FERROELECTRIC MATERIAL Alvin A. Snaper, 2800 CameoCircle, Las Vegas, Nev. 89107 Filed Sept. 15, 1972, Ser. No. 289,480Int. Cl. C23c /00 US. Cl. 204-192 8 Claims ABSTRACT OF THE DISCLOSURERecrystallized ferroelectric material is deposited by sputtering on aheated substrate by applying radio-frequency voltage to a target holdersupporting a target of ferroelectric material held in proximity to thesubstrate within a chamber which is evacuated except for the presence ofa small amount of inert gas, and optionally with a small amount ofoxygen. The radio-frequency voltage causes ions of the gas to strike thetarget material with such force as to break off molecules or atoms whichbond themselves to the substrate. The target holder is provided withfluid coolant channels which protect the target from deterioration.

This invention relates to the sputtering deposition of recrystallizedferroelectric material and more particularly to a method and apparatuscapable of producing such a deposit.

Ferroelectric materials such as gadolinium titanate, gadoliniummolybdate, gadolinium niobate, barium titanate, and bismuth titanatehave been known. Such ferroelectric materials when in crystal formpossess birefringent properties permitting their use as lightmodulators, enabling them to affect transmission of light in variousways, for example, the production of color effects, or the passing,reducing or blocking of light transmission in ways dependent on suchfactors as the physical dimensions of the material, the application ofan electric field or voltage and the relationship of the light to thematerial.

It has heretofore been known to deposit ferroelectric materials but thedeposited material has been amorphous in structure.

It has heretofore been known to create crystals of ferroelectricmaterial. One such method has been a seeding process by which arelatively large crystal is grown from a much smaller seed crystal.Another prior known method has been the so-called flux process, by whichsmall pieces of crystal material are obtained from a flux.

It is an object of the present invention to provide a method and meansfor obtaining recrystallized ferroelectric material by deposition.

A related object is to create such a recrystallized ferroelectricdeposit by sputtering on a substrate or the like.

A further related object is to provide a fluid-cooled target holder forcarrying the ferroelectric material used as the target in the sputteringoperation.

The invention is carried out by the application of radiofrequency (R.F.)electrical power to a target holder containing a ferroelectric targetmaterial, in a high vacuum to which there have been introduced minoramounts of inert gas, preferably a relatively heavy inert gas such asargon, xenon, neon or krypton or mixtures thereof. The RF. energy actingupon the gas creates a plasma. A heated substrate or the like spacedsomewhat from the target material receives a layer of recrystallizedferroelectric material bonded to it due to the sputtering of thismaterial from the target by the actions of ions of the plasma strikingthe target material in response to the radio frequency energy. Althoughlighter inert gases such as nitrogen are usable, they are not asefficient as the heavier ones since ions of the lighter gases do notpossess as much kinetic energy as ions of the heavier gases.

3,838,031 Patented Sept. 24, 1974 In the sputtering deposition, there isa high temperature build-up which creates risk of the target materialbreaking, cracking, or exploding, or becoming contaminated. Under suchcircumstance, it is desired to avoid or minimize this risk by providingcooling for the target material. According to a feature of theinvention, the target holder may be cooled by fluid, ordinarily water.Novel aspects reside in the configuration and arrangement of the coolantchannels at the target holder.

Recrystallized ferroelectric material can be used in such equipment as:computers or data processing systems, such as that of Snaper Pat.3,348,217, issued Oct. 17, 1967; polarized film structures such as thoseof Snaper Pat. 3,- 376,135, issued Apr. 2, 1968; color-producing tubessuch as that illustrated in Snaper Pat. 3,391,296, issued July 2, 1968;electro-optical devices such as that illustrated in Snaper Pat.3,445,666, issued May 20, 1969; color-producing equipment such as thatillustrated in Snaper Pat. 3,- 488,106, issued Jan. 6, 1970; and planarrandom access ferroelectrie computer memories such as that illustratedin Snaper et al. Pat. 3,675,220, issued July 4, 1972. These are onlysome of the many possible uses for the material.

The invention will be better understood from the following detaileddescription and the accompanying drawing, of which:

FIG. 1 is a cross-section view showing equipment for carrying out an RF.sputtering deposition according to this invention;

FIG. 2 is an elevation view in cross-section showing a target holderwhich may be used in the arrangement of FIG. 1;

FIG. 3 is a front view of a portion of the head of the target holder ofFIG. 1;

FIG, 4 is a side view of the portion shown in FIG. 3;

FIG. 5 is a top view of the portion shown in FIGS. 3 and 4;

FIG. 6 is a side view of another portion of the head of the targetholder of FIG. 1; and

FIG. 7 is a bottom view of the portion shown in FIG. 6.

Referring to FIG. 1, there is shown a vacuum bell jar I mounted with anO-ring seal In on a base member 2 provided with an opening 3 forconnection to the inlet of a vacuum pump. The bell jar will ordinarilybe circular in its top view, although it may be of a different shape. Ableed conduit 4 provided with a bleed valve 5 is adapted for connectionto a source of gas which it may be desired to introduce into theevacuated bell jar. The base 2 is provided with a centrally locatedopening 6 to the wall in which there is attached an upstanding hollowtubular member 7 of an electrical conducting material extending upwardlyinto the bell jar. The upper end of this tubular member is provided withan annular head 8 having an outwardly and upwardly beveled surface 9which is attached to a corresponding beveled surface 10 of a targetholder 11 through an electrical insulating member 12 sealed to both thetarget holder and the head 8. The top surface of the target holder isflat and horizontal and preferably circular in its top view. The targetholder has attached to its underside a stem 13 extending down through,and spaced from, the tubular member 7 to the exterior of the bell jar.The members 2, 7, 8, 11, and 13 are of electrical conducting material,ordinarily metal, and the material of the bell jar 1 is electricallyinsulating. There is bonded to the top of the target 11 a layer offerroelectric target material 14 which will ordinarily be in theamorphous form. The bonding may be done by use of an electricallyconductive bonding material such as 0 a silver conductive paste and theapplication of heat and There is supported within the bell jar at aposition above the target material a horizontally disposed plate ofelectrical-conducting material, ordinarily metal, pro vided with anopening 16 at a position above the target material 14, There issupported within this opening a substrate 17, ordinarily of an opticallytransparent electrically-insulating material which should ordinarily becompatible with transparent conductors and with opaque high dielectricmaterials. Typical substrate materials can be Pyrex glass or quartz orthe like. Before a sputtering operation, the substrate should be madevery clean, which can be done by cleaning ultrasonically and baking in ahigh temperature oven, or by reverse sputter etching. Then the substrateis mounted to a matrix mask 18, the configuration or position of whichwill determine the area or areas of the deposition on the substrate.Above the substrate, there are supported heating elements 18 ordinarilyin the form of high-temperature quartz lamps which direct their heatdown upon the top of the substrate.

To provide radio frequency power for the sputtering operation, a source20 of radio frequency voltage is applied over respective conductors 21and 22 between the grounded elements 2, 15 and the electrode 13', whichthus acts as an antenna radiating electromagnetic energy into the belljar.

To perform a sputtering operation with the system of FIG. 1, a highvacuum pump is attached at the entrance 3 to the bell jar and vacuumpumped to a very high vacuum of about 10- to 10' torr while the heaters19 are turned on to preheat the substrate 17 to a high temperaturewhich, though not critical, should be between about 200 degrees and 600degrees C. When the desired temperature and vacuum levels are reached,the vacuum pump is throttled to a partially closed condition by means ofa valve 23 at a vacuum port 3 Opening and the inert gas, ordinarilyargon, is bled into the chamber through tube 4 past valve 5. The amountof the inert gas such as argon, xenon, or neon or the like bled into thesystem is not critical, and may be between 1.0x 10 torr and 40x10 torr.Argon is preferred over xenon or neon for the inert gas, as argon isusually less costly.

It may, in many instances, be desirable to bleed into the chamber alongwith the inert gas, some amount of oxygen to act as a control factor. Areason for this is that ferroelectric materials commonly contain oxygenin the binder, some more than others. If a ferroelectric materialdeficient in oxygen relative to others is being used for the sputtering,it is found desirable to bleed into the chamber an amount of oxygensufiicient to make up the oxygen deficiency in the material. In the caseof the use of gadolinium molybdate as the ferroelectric material andwith the use of argon as the inert gas, it has been found beneficial tointroduce with the inert gas an amount of oxygen between about 1.0x 10'torr and about 10.0)(10' torr, the precise amount of oxygen not beingcritical.

After this evacuation and gas bleeding operation has been performed, andwhile the heating elements 19 remain operating, the radio frequencyelectrical energy from source 20 is turned on to apply it to theelectrode 11, 13, the radiation from which thereby ionizes the argon (orother inert) gas. The negative cycle portions of the RP. energy causepositive ions, represented in FIG. 1 by circles 55 containing plussigns, to strike the target material, thus knocking olf particles in theform of atoms and molecules (probably mostly molecules) of the targetmaterial toward the under surface of the substrate. These particlesstriking the substrate through the mask 8 have enough energy to bondthemselves into the substrate.

The radio frequency energy must be of at least a high enough frequencyto create the plasma and this minimum frequency is believed to be around5 megacycles per second. A normal range would be about 5 to 25megacycles per second, which is below the microwave range. A proper R.F.power level would be between about 1 and 10 watts per square centimetersurface of the target material. The

time of the sputtering operation will be dependent on the thickness ofthe recrystallized ferroelectric layer desired on the substrate, thelonger the time, the greater the thickness.

A sputtering deposition using gadolinium molybdate as the targetmaterial was performed under the following test condition:

Substrate material: quartz; target to substrate distance 1% inches;argon gas pressure 4X10 torr; oxygen gas pressure 1.0x 10' torr; quartzsubstrate temperature 400 centigrade using quartz heating lamps.Starting with a very clean vacuum system and chamber, the system waspumped onto a very high vacuum, and then the vacuum pump Was throttledas the argon and oxygen were backfilled into the chamber to theforegoing pressure. During this process, the heating lamps were kept on,and holding the substrate temperature at 400 C. At this time, the RF.power of a frequency of 12.7 megacycles per second was applied to thetarget, igniting an RF. plasma. The radio frequency power was increasedslowly to a power level of 7 watts per square cm. of target materialsurface area. Sputtering under these conditions for approximately 8hours achieved a thickness of 20,000 angstroms (7.874 l0- inches), whichis a typical thickness. The radio frequency power and the substratetemperature were then both decreased slowly for four hours until thesubstrate temperature dropped to 60 C., whereupon the system was shutdown. The substrate was then removed and placed in a furnace, whichheated it to a temperature of 700 C. The substrate was then cooledslowly, thereby removing all stress from the crystalline material whichhad been sputtered onto it. It was found that the sputtered material wasof gadolinium molybdate crystalline structure and had excellentbirefringence.

For any selected RF. power level the thickness of the deposit is aboutproportional to the time of the sputtering.

Inasmuch as the uses to which recrystallized material depositedaccording to this invention will ordinarily be put, will generallyinvolve its effects in response to the application of an electric field,there will generally be applied to the substrate a substance which actsas an electrical conductor. This may be done, for example, by vacuumdepositing a film of conductive stannic oxide on the substrate prior tothe ferroelectric sputtering operation. Such a conductive stannic oxidedeposit may be made in a well-known manner. Then when the substratebearing the conductive stannic oxide film on its undersurface is placedin the bell jar with the matrix mask, the crystalline ferroelectriclayer from the sputtering will bond to the parts of the substrate notcovered by the conductive stannic oxide film and also over theconductive stannic oxide film. Thus, the conductive stannic oxideconstitutes an electrically conductive surface adjacent to theferroelectric material to which an electrical lead or conductor may bebrought in a suitable manner to apply voltage over the ferroelectriclayer. Such a stannic oxide film may be formed over the entireferroelectric layer or, if desired, only over part of it, as forexample, in strips, according to the effects which are desired. Thestannic oxide is not only electrically conductive, but is transparentand compatible with such substrate material as Pyrex glass and quartzand is compatible with ferroelectric material which is a ceramic type ofdielectric. Stannic oxide is mentioned merely as an example of acompatible transparent conductor which may be used for the purpose. Thinmetallic films and other materials may also be utilized.

The ferroelectric deposit has been referred to herein as recrystallizedferroelectric material, which is the term usually applied to it eventhough the ferroelectric material constituting the target is amorphousand not crystalline.

FIGS. 2 to 7 illustrate a fluid-cooled target holder which can be usedas the target holder of FIG. 1. Referring particularly to FIG. 2, thestem 13 constituting the RF. elec trode is in the form of a hollowconduit 30, terminating at its upper end in a flanged collar 31 havingattached to it a circular head 32 of greater diameter than that of tube30.

The target holder 11 comprises a plate 33, shown in detail in FIGS. 3, 4and 5, and an upper plate 34 shown in detail in FIGS. 6 and 7. FIG. 6 isa front elevation view and FIG. 7 is a bottom view, of the upper plate34, from which it is seen that the plate is flat and circular with upperand lower surfaces which are horizontal and with a side peripheralsurface which is beveled inwardly in a downward direction from the top.The lower surface is flat and horizontal excepting for an arrangement ofconnected generally circular grooves 35 machined into the plate from thebottom as shown in FIG. 7. This groove arrangement comprises a centralrecess 35a located at the central axis of the plate, around which arerecesses arranged in generally concentric circles. The central recess35a communi cates through an opening 36 into the end of the innermostconcentric circle 35b, the end of which communicates through an opening37 into the next concentric circle 350, the end of which in turncommunicates through passageway 38 into the outermost concentric circle35d. The end of the outermost concentric circle connects with a circularrecess 39.

The lower plate 33 of the target holder 11, illustrated in detail inFIGS. 3, 4, and 5 is a flat circular plate of about the same thicknessas plate 34, having horizontal upper and lower surfaces with a perpheralsurface beveled at the same angle as plate 34. The diameter of the uppersurface of plate 33 coincides with the diameter of the lower surface ofplate 34. Plate 33 has formed through it at its central axis, an opening40 of somewhat smaller diameter than recess 35a of plate 34, and adaptedto register with recess 35a.

The underside of plate 33 has machined into it a concentric circularrecess 41 shown dotted in FIG. 3, and also in FIG. 5, the top view ofthe plate. From one side of recess 41, there is formed a channel 42communicating from the recess 41 to a circular opening 43 extending fromthe top of the plate to meet the end of channel 42. Plates 33 and 34 arefastened together as by welding around the beveled periphery.

As shown in FIG. 2 the electrode 13 is assembled to target holder 11 byuse of a hollow pipe or conduit 44 extending through and spaced from theinner wall of tube 30. The upper end of pipe 44 is provided with outsidethreads 45 which threads into corresponding internalthreads 46 inopening 40 of plate 33, and also to threads 47 at opening 48 of head 32.

To assemble the target holder to the electrode 13 as shown in FIG. 2,the head 32 of the electrode will be threaded onto the threads 45 ofpipe 44 until the end of the threading is reached to secure the head 32firmly on the pipe. This will leave a substantial part of the pipethreading protruding above head 32, so that the threads 46 of plate 33of target holder 11 can then be threaded until the plate 33 assumes aposition firmly against the top of head 32. When thus assembled, theconcentric recess 41 of plate 33 aligns with the vertically extendingbores 49 of head 32 which communicate with the annular passage 50between pipe 44 and tubular electrode 30. In this assembly, the passage50 within pipe 44 is in communication with recess 35a of the upper plate34. Owing to the close fit between plates 33 and 34, there is no fluidleakage from the concentric channels 35b, 35c and 35d between these twoplates. To insure against fluid leakage be- 6 tween head 32 and thebottom of plate 33, the head 32 is provided with concentric recesses 52and 53 containing resilient O-rings to act as seals.

Fluid, ordinarily a liquid, preferably water, can be circulated throughthe target holder in the direction of arrows 54, by a suitable pump orpressure means. Thus, fluid is forced upwardly through conduit 44 intorecess 35a from which it passes through the concentric recesses 35b, 35cand 350! to channel 42 and downward through the concentric passage 50.

It will be understood that the embodiments of the invention illustratedand described herein are given by way of illustration and not oflimitation, and that modifications or equivalents or alternatives withinthe scope of the invention may suggest themselves to those skilled inthe art.

I claim:

1. Method of depositing recrystallized ferroelectric material whichcomprises applying radio frequency power to a target comprisingferroelectric material in proximity to a substrate heated to atemperature between 200 C. and 600 C. within an enclosure substantiallyevacuated, but containing some inert ionizable gas, said voltage beingof a frequency and intensity sufficient to create a plasma comprisingions derived from the gas, causing at least some of said ions to strikethe ferroelectric material with such force as to sputter particles of itto the substrate, with sufficient velocity to bond it to the substratein crystalline form.

2. Method according to claim 1 in which the sputtered particles comprisemolecules or parts thereof, or both.

3. Method according to claim 1 in which the frequency is between about 5and 25 megacycles per second.

4. Method according to claim 1 in which the inert gas is selected fromthe group consisting of argon, xenon, neon, and krypton, and mixturesthereof.

5. Method according to claim 1 in which the inert gas is present in anamount between about 1.0 10- torr and 40 X 10* torr.

6. Method according to claim 5 in which oxygen is present in an amountbetween about 1.0 10 torr and 10.0X l0 torr.

7. Method according to claim 1 in which the ferroelectric material isselected from the group consisting of gadolinium titanate, gadoliniummolybdate, gadolinium niobate, barium titanate and bismuth titanate.

8. Method according to claim 1 in which the ferroelectric material isgadolinium molybdate.

References Cited UNITED STATES PATENTS 8/1972 Vogel 204-192 8/1970Davidse et al. 204298 X 3,730,867 5/1973 Albers, Jr. et al. 204-298 X3,707,452 12/1972 Lester et al. 204-298 X JOHN H. MACK, Primary ExaminerD. R. VALENTINE, Assistant Examiner U.S. Cl. X.R.

